Extraction of hexavalent plutonium from aqueous acidic solutions with ethyl sulfide



EXTRACTION F HEXAVALENT PLUTONIUM FROM AQUEOUS A'CEDIC SOLUTIONS WITH 'ETHYL SULFIDE Glenn T. Seaborg, Chicago, 111., assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing, Filed May 1, 1945, Ser. No. 591,410

2 Claims. (Cl. 23-445) The present invention relates to the solvent extraction of heavy metal compounds, and particularly to the extraction of a compound of a transuranic element from an aqueous solution by means of an organic solvent.

An object of this invention is to provide a method for the separation and purification of a tranuranic element contained in aqueous solutions of said element and various other elements of lower atomic weights.

Other objects and advantages of this invention will be evident from the following description.

The term element 94 is used in this specificationto designate the element of atomic number 94, which is also referred to herein as plutonium, symbol'Pu. It has recently become known that various isotopes of element 94 can be prepared by a number of different nuclear processes. Thus, the plutonium isotope of mass 238, referred to as 94 can be prepared by deuteron bombardment of the uranium isotope 92 produce 93 followed by beta decay of the 93 to 94 (Theisotope93 'is the mass isotope of the element of atomic number 93, referred to as neptunium, symbol Np.) The plutonium isotopes 94 and 94 can be prepared by neutron bornbardment of the uranium isotope of mass 238:

N-eutronic reactors for the production of element 94 from natural uranium by a self-sustaining chain reaction have recently been developed. One of the isotopes occurring in natural uranium is 92 which is present to the extent of about 0.71% by weight. When this isotope is bombarded by slow neutrons, preferably of thermal energies, it undergoes fission and releases, on an average, about two neutrons per fission in addition to fission fragments of relatively low atomic weights. The isotopes 94 and 94 can also undergo fissionrwhen bombarded by neutrons, and these fission processes also release an average of about two neutrons per neutron absorbedin the fission reaction. Thus, in a 'natural-uram'um neutronic reactor, the excess neutrons released by fission are suflicient to maintain productioniof element 94 through neutron absorption by the predominant uranium isotope 92 The concentration of plutonium so produced is generally small, rarely being above one percent by weight of the uranium and usually being substantially below this concentration.

Element 94 may suitably be separated from massesof uranium which have undergone neutron bombardment for periods of time sufii'eient to produce recoverable amounts of plutonium, but which may be-insufiicient to destroy more than a minute fraction of the total uranium through fission of 92- andneutron absorption by 92 It may be desired to recover the plutoniumcontent of such uranium masses having plutonium concentrations below 1 part per thousand and even below 1 art per million parts of uranium.

Neutron-bombardeduranium, or a neutron-bombarded uranium compound, will contain, inaddition to plutonium and unreacted uranium, various radioactive fission prodnets in a. total concentration of the same-order of magni- United States Patent 9 tilde as the plutoniumconcentrati'om The ori'gina lfrag- 1 ments produced by fission are highly unstable radioactive isotopes of short half lives which rapidly decay to longerlived isotopes. The fission products which are present after practicable aging periods will usually range in atomic numbers from about 32 to about 64 and in atomic masses from about 77 to about 158. These fission products coinprise largely a light group in which isotopes of Sr, Y, Zr, Nb, and Ru are the most dangerously radioactive, and a heavy group in which isotopes of Te, I, Cs, Ba, La, Ce, and Pr are the most dangerously radioactive. i

In the recovery of plutonium from neutron-bombarded uranium, namely in the initial separation of plutoinum from the bulk of the unreacted uranium and fission products, in the further decontamination of the plutonium by separation from residual radioactive fission products, and in the final concentration, purification, and recover; of plutonium, it is desirable to effect the plutonium separations from aqueous nitric acid or other suitable aqueous medium. Various precipitation methods have been suggested for the separation of plutonium from such aqueous solutions. These methods, however, have generally entailed the use of large amounts of auxiliary carrier precipitates and the handling of very large volumes of solutions. "I

In accordance with the present invention, plutonium compounds may be separated from aqueous nitric acid solutions, aqueous. hydrochloric acid solutions, or other suitable aqueous media, byextraction into organic solvents of the class hereinafter described. This solvent extraction procedure may be employed for the separation of plutonium from aqueous solutions in any phase of the above-mentioned procedure for the recovery of plutonium from neutron-irradiated uranium. In addition, the solvent extraction procedure of this invention may be used for the separation of plutonium compounds from aqueous solutions other than those derived directly from neutron} irradiated uranium. Such solutions may range from waste liquors containing only traces of plutonium to relatively concentrated solutions of pure plutonium compounds.

The aqueous solutions from which plutonium may be extracted in accordance with the present invention suitably contain plutonium ions of at least the +4 valence state, said ions being substantially free from strong cornplexing agents. Preferably, no ion should be present in the aqueous solution which forms ionic complexes with plutonium more strongly than does the nitrate ion. The hydroxyl ion, and the anions of acids which ionize in aqueous solutions to a substantial-1y smaller degree than nitric acid, are particularly disadvantageous from the standpoint of complexing tetravalent plutonium. Thus, hydroxyl, sulfate, phosphate, fluoride, and oxalate ions tend to complex tetravalent plutonium sufficiently to decrease its extractability into organic solvents. It is therefore desirable to minimize the concentrations of free ions of this class in the aqueous solutions to be extracted. Most of the complexing ions can be excluded in the preparation of the aqueous solutions. Alternatively, an interfering ion may itself be complexed by another ion, or its concentration as a free ion may otherwise' be minimized by control of the concentration of another ion which can combine with it. Thus, the fluoride ion can be complexed by zirconyl ion, and the hydroxyl ion can be suppressed by'hydrogen ion, to reduce their interference with the extraction of tetravalent plutonium.

The extraction of hexavalent plutonium is relatively free from interference by hydroxyl ion. Hexavalen't plutonium can be extracted from aqueous solutions of any hydrogen ion concentration sufiicient to prevent the precipitation of a basic plutonium compound. An aqueous solution of plutonyl nitrate may thus be extracted without the necessity of free nitric acid in the solution. In the case of tetravalent plutonium, on the l r 3 other hand, it is desirable to maintain free acid in the aqueous solution. Solutions of plutonous nitrate for extraction with organic solvents should have a pH not ,substanially above 2.5, and preferably should have a concentration of free nitric acid of at least 1 N.

In order to improve the distribution of plutonium between the aqueous and organic phases, it is generally desirable to incorporate a salting-out agent in the aqueous solution. A salting-out agent, for this purpose, has the same characteristics as a salting-out agent for previously known solvent extraction processes, i.e., high solubility in the solution to be extracted and low solubility in the extract phase. presence of high concentrations of any substances which tend to dissolve in the solvent phase more readily than the plutonium, is to be avoided. If such a condition is permitted to occur a salting back of any plutonium present in the solvent layer takes place, thus tending to greatly decrease the efliciency of this process.

salting-out agents for this purpose are NaNO3 Ca(NO3)3 KNO3 SI' 2 LiNO Mg(NO NH4NO3 La M 3)2 a)a The concentration of the salting-out agent which is deof the cation and the concentration of the common anion due to any free acid in the solution. In the caseof l N nitric acid solutions, it is desirable to employ a concentration of a univalent nitrate of at least 3 M, and preferably Likewise it will be apparent that the 1 stable in any particular case will depend on the valence 5-10 M. Equivalent concentrations of polyvalent nitrates may be employed at the same acid concentration, and the salt concentration may suitably be increased or decreased with decrease or increase in acid concentration.

The extraction agents which are suitable for use in this process comprise normally liquid organic solvents which are substantially immiscible with the aqueous solution to be extracted and which contain at least 1 atom capable of donating an electron pair to a coordination bond. Such solvents suitably comprise water-immiscible organic compounds containing .an oxygen, sulphur, or nitrogen electron-donor atom. It will be evident, however, that most nitrogen-containing organic compounds of this type are basic in nature and will be unsuitable for the extraction of acidic aqueous solutions. Such compounds may be used, if desired, to extract substantially neutral solutions containing small amounts of hexavalent plutonium. Most organic solvents containing oxygen or sulphur donor atoms may be used for the extraction of acidic aqueous solutions as well as neutral solutions, and the oxygenated organic solvents are the preferred extractants for use in the present process.

Although most normally liquid organic compounds containing an atom with excess electrons, such as oxygen, sulphur, or nitrogen, are capable of forming a coordination bond, it will be evident to those skilled in the art that certain molecular structures can interfere with this electron-donating property. Electron-attracting constituents such as halogen atoms can offset the electron-donating property of an atom such as oxygen, if present in suflicient number and proper relationship to the donor atom. For this reason it is preferable to employ compounds containing only carbon, hydrogen and electrondonor atoms. It will also be apparent that certain molecular configurations can give rise to steric hindrance which may interfere sufficiently to prevent the formation of coordination bonds. Tertiary carbon atoms adjacent an electron-donor atom and long chains of non-donor 4 atoms linked to a donor atom are especially undesirable in this respect. The preferred solvents are those in which the donor atom is linked to a hydrogen atom or to nontertiary carbon atoms and in which at least one component linked to the donor atom contains less than fou consecutive non-donor atoms.

The following are examples of suitable solvents for use in the present process:

jEthyl ether 2-phenox'yethanol Z-benzyloxyethanol Z-(B-ethylbutoxy) ethanol l ,2-diethoxyethane Methylisobutylcarbinol Methyl ethyl ketone Methyl amyl ketone Methyl isobutyl ketone Mesityl oxide In employing any of the solvents of the above class in the present process, previously known extraction procedures andapparatus may be employed. The extraction may be effected by batch, continuous batch, batch counter-current, orcontinuous counter-current methods. The most efiicient extraction is obtained in continuous counter-current operation. The usual types of extraction equipment .and.the usual operating procedures may be employed when 'e'ffecting the. present extraction in this manner. Thus, for example, if the solvent is lighter than water, satisfactory extraction in accordance with this procedure may be obtained by the use of a packed column with. aqueous feed at an intermediate point, solvent feed at the bottom of the column, and solvent drawofi at the top of the column. The top section of such a column may serve as a stripping section, and an auxiliary stripping medium charged to the top of the column. With certain obvious changes, solvents heavier than water can be employed with substantially equally good results.

The plutonium may be recovered from the solvent extract phase by any suitable procedure such as evaporation of the solvent, crystallization with an isomorphous crystalline carrier, re-extraction with another immiscible solvent,- or adsorption on a solid adsorbent.

It is apparent that the procedures described above may be applied to the extraction of plutonium from solutions containing various impurities of lower atomic weight which have partition coefficients between solvent and aqueous phases lower than that of plutonium. The purification effected by this procedure is especially advantageous in separating plutonium from the fission products contained in solutions derived from neutron irradiated uranium. This procedure is also useful in simultaneously extracting both plutonium and uranium from solutions of neutron bombarded uranium. Repeated extractions with one or more solvents may be used to eifect purification and concentration of plutonium to an extent sufficient to enable final recovery of a pure compound of plutonium.

The present invention will be further illustrated with particular reference to certain preferred classes of solvents.

One preferred class of solvents for use in the present invention comprises ethers containing at least one oxygen atom capable of donating an electron pair to a coordination bond. The most desirable compounds of this class are the saturated aliphatic ethers, and it has been found preferable to employ a dialkyl ether containing at least one oxygen atom which is attached by an ether linkage assogsra 5 to non-tertiary aliphatic carbon atoms, at least one component linked to said oxygen atom containing less than four consecutive non-donor atoms. The following examples illustrate the use of solvents of this class:

Example I A solution of PuO in 1 M HNO 10 M NH NO was charged into an extractioncolumn and continuously extracted with approximately 5.4 times its volume of ethyl ether. At the conclusion of the extraction, the extract and raflinate were analyzed for plutonium content on the basis of the alpha radiation of the initial charge and of the resulting solutions. The plutonium distribution was found to be as follows:

Percent Extract 91.2

Raflinate 8.3

Loss 0.5

Example 3 Uranyl nitrate hexahydrate, which had been subjected to neutron irradiation from 50 milliampere hours deuteron bombardment of beryllium and aged for 80 days, was melted, acidified with 70% nitric acid to 0.66 M HN made 0.02 M in K Cr O- with solid potassium dichromate, and held at 60 C. for about one hour to effect oxidation of the plutonium to PuO The solution was then cooled to room temperature and extracted with approximately 5.3 times its volume of ethyl ether, in a single stage batch extraction. The extract was found to contain 86% of the plutonium content of the aqueous charge, together with a substantially greater percentage of the uranium content of the charge, but only 8% of the fission products.

Example 4 A solution of Pu+ in 1 M HNO -l0 M NH NO was agitated with an equal volume of ethyl ether until the extraction equilibrium was attained. The phases were then separated and the extract was analyzed for plutonium content on the basis of its alpha radiation and that of the original aqueous solution. The plutonium content of the extract was found to be 35% of that in the initial charge.

The substitution of normal propyl or isopropyl ethers for the ethyl ether in the above examples will result in less advantageous distribution coefli'cients, but adequate extraction is obtainable in eflicient counter-current operation.

The sulphur analogues of the ethers are also useful in in this process, and the following example illustrates the use of a solvent of this type:

Example 5 A. solution of PuO in 1 M nNo o M NI-I NO was agitated with an equal volume of ethyl sulfide until -'the extraction equilibrium was attained. The phases class are saturated glycol. ethers containing at least one oxygen atom which is attached by an ether linkage to a non-tertiary aliphatic carbon atom, at least one component linked to said oxygen atom containingless than four consecutive non-donor atoms. The preferred types of ethers of this class are the monoglycol monoalkyl ethers, polyglycol monoalkyl ethers, monoglycol dialkyl ethers, and polyglycol dialkyl. ethers. The following are examples of the use of solvents of this class:

Example 6 A solution of PuO in 1 M HNO 10 M NH NO was agitated with an equal volume of 1,2-diethoxyethane until extraction equilibrium was attained. The phases were then separated and the extract was analyzed for plutonium content on the basis of its alpha radiation compared with that of the original aqueous solution. The plutonium content of the extract was found to be 96% of that in the initial charge.

Example 7 A solution of Pu+ in l M HN0 10 M NH NO was charged into an extraction column and continuously extracted with approximately 6 times its volume of 2(B-ethylbutoxy)ethanol. At the conclusion of the extraction the extract and raflinate were analyzed for plutonium content. The plutonium distribution was found to be as follows:

Percent Extract 83.6

Raflinate 14.4 Loss 2.0

Example 8 A solution of Pu in 0.6 M UO (NO 1 M HNO 10 M NH NO was agitated with an equal volume of 2( 3-ethylbutoxy)ethanol until the extraction equilibrium was attained. The phases were then separated and the distribution of plutonium and uranium between the aqueous and organic phases was determined. The percentage extraction of each of the metals from the initial aqueous solution was found to be approximately as follows:

. Percent Plutonium g r 60 Uranium 63 Example 9 A solution of Pu in 1 M HNO 'l0 M- NH NO was agitated with an equal volume of Z-benzyloxyethanol. After extraction equilibrium had been attained, the phases were separated and the extract was analyzed for plutonium content. It was found that the extraction was approximately 48% complete.

Another preferred class of solvents for use in the present extraction process comprises normally liquid organic solvents containing at least one carbonyl oxygen atom capable of donating an electron pair to a coordination bond. This class includes carboxylic acids, esters, aldehydes and ketones. The most desirable solvents of this group comprise aliphatic ketones and especiallydialkyl ketones having a carbonyl group attached to at least 1 non-tertiary carbon atom. The following are examples of the utilization of solvents of this class in the present process:

.Example 10 A solution of Pu in 1 M HNO -10 M- NH NO was agitated with an equal volume of methyl ethyl ketone until the extraction equilibrum was attained. The phases were then separated and the extract was analyzed for plutonium content on the basis of its alpha radiation and that of the original aqueous solution. The plutonium content of the extract was found to be 78.9% of that in the initial charge.

Example 11 A solution of Pu+ in 8 M HCl was agitated with an equal volume of methyl isobutyl ketone. After extraction equilibrium had been attained, the extract was analyzed for plutonium content. The extraction was found to be 91% complete.

j Examplel3 A lanthanum fluoride precipitate containing plutonous fluoride (Pu was dissolved inaqueous zirconyl nitrate and the resulting solution was made 3 M with respect to nitric acid and M with respect to ammonium nitrate. The concentrations of zirconium, lanthanum and plutonium in this solution were approximately 18.6, 6.2 and 0.0001 g. per liter, respectively.

The above solution was charged continuously to a packed extraction column at a point approximately 0.4 of the column length below the top of the column. Methyl isobutyl ketone was charged continuously to the bottom of the column at a rate approximately double the charge rate of the plutonium solution, and an aqueous stripping solution, comprising 3 M I-INO -5 M NH NO was continuously charged to the top of the column at a rate approximately one-half the charge rate of the plutonium solution. The total charge to the column, per hour, was approximately 0.9'of the column hold-up.

After reaching equilibrium, the methyl isobutyl ketone extract, which was continuously withdrawn from the top of the column, and the spent aqueous solution, which was continuously withdrawn from the bottom of the column, were analyzed for plutonium content. The extract was found to contain approximately 99.75% of the plutonium in the charge solution, and the spent aqueous solution contained the residual 0.25%.

Example 14 A solution of Pu+ in 1 M HNO M NH NO was agitated with an equal volume of acetophenone until the extraction equilibrium was attained. The phases were then separated and the extract was analyzed for plutonium content on the basis of its alpha radiation and that of the original aqueous solution. content of the extract was found to be 89% of that in The plutonium the original charge.

Example 15 'especially nitroalkanes. The following are examples of the use of nitrohydrocarbons in the present process:

. Example 16 i A solution of Pu+ in 1 M HNO -10 M NH NO' 'was agitated with an: equal volume of nitromethane until the extraction equilibrium was attained. The phases were then separated and the extract was analyzed for plutonium content. The extraction was found to be approximately 58% complete.

Example 17 A solution of PuO in saturated aqueous lithium nitrate was agitated with 1.5 times its volume of nitromethane until the extraction equilibrium was attained. The phases were then separated, and the extract was found to contain approximately of the plutonium content o'f the original aqueous solution.

Example 18 A- solution of Pu0 in 1 M HNO --10 M NH NO was agitated with an equal volume of nitroethane until the extraction equilibrium was attained. The phases were then separated and the extract was analyzed for plutonium content. The extraction was found to be approximately 6l% complete.

Example 19 A solution of PuO in saturated aqueous lithium nitrate was agitated with 1:5 times its volume of 1- nitropropane until the extraction equilibrium was attained. The phases were then separated and the extract was found to contain approximately 81% of the plutonium content of the original aqueous solution.

Example 20 A solution of PuO in 1 M HNO 10 M NH N0 was agitated with an equal volume of nitrobenzene. The phases were then separated and the extract was analyzed for plutonium content. The extraction was found tobe approximately 28% complete.

It is to be understood, of course, that the above examples are merely illustrative and do not limit the scope of the present invention. Other solvents of the class previously described may be substituted for the specific solvents of these examples and the procedures employed may be modified in numerous respects within the scope of the foregoing description. In general, it may be said that the use of any equivalents or modifications of procedure which would naturally occur to those skilled in the art is included in the scope of this invention. Only such limitations should be imposed on the scope of this invention as are indicated in the appended claims.

This is a continuation in part of copending application, U.S. Serial No. 481,660, filed April 3, 1943, now US. Patent 2,811,415, issued October 29, 1-957, and all subject matter therein not inconsistent with the subject matter herein is incorporated in the present application by reference.

What is claimed is:

1. A process of separating hexavalent plutonium values from an aqueous nitric acid solution also containing ammonium nitrate as a salting-out agent, said process comprising contacting said solution with ethyl sulfide and separating a phase of ethyl sulfide containing said plutonium values fiomthe aqueous solution.

2. The process of claim 1 wherein the aqueous solution contains nitric acid in a concentration of l M, ammonium nitrate in a concentration of 10 M and wherein equal volumes of aqueous solution and ethyl sulfide'are used.

References Cited in the file of this patent 5 UNITED STATES PATENTS 2,227,833 Hixson et al. Jan. 7, 1941 1,811,415 Seaborg Oct. 29, 1957 

1. A PROCESS OF SEPARATING HEXAVALENT PLUTONIUM VALUES FROM AN AQUEOUS NITRIC ACID SOLUTION ALSO CONTAINING AMMONIUM NITRATE AS A SALTING-OUT AGENT, SAID PROCESS COMPRISING CONTACTING SAID SOLUTION WITH ETHYL SULFIDE AND SEPARATING A PHASE OF ETHYL SULFIDE CONTAINING SAID PLUTONIUM VALUES FROM THE AQUEOUS SOLUTION. 